Airborne platforms carrying electro-optical (EO) sensors for such tasks as target acquisition, identification, guidance, and the like, are generally provided with a transparent dome to protect the optical system. In particular, projectiles, such as missiles, interceptors, guided projectiles, bombs, rockets, shells and sub-munitions, typically have the dome in the front end. Behind this dome, and within the body of the projectile, an EO seeker is provided for capturing electro-magnetic radiation (EMR) from the target, and conveying target information (e.g. bearing or images) to a guidance system, which, in turn, guides the projectile to an object or point within the captured images. Aircraft such as planes or helicopters may be provided with a directed infrared countermeasures (DIRCM) system to jam a missile seeker. This system may be mounted on the belly, tail section or elsewhere on the aircraft behind a protective dome. The dome is generally made of a transparent material that can sustain the aerodynamic and thermal stresses it may experience during missile or aircraft flight. In many conventional applications the dome is made of sapphire.
The size of the field of regard (FOR) obtainable by the EO seeker depends on the spanning angle of the dome used. The term “spanning angle” when used herein refers to the actual angular portion that the dome spans without vignetting with respect to a full sphere whose spanning angle is 360°. The angle measured from the longitudinal axis through the center of the dome to the edge of the FOR is one-half the spanning angle and is referred to as the “look angle.” Conventional missile domes, such as sapphire domes, are made of, at most, approximately half a sphere size. Therefore, when a conventional optical seeker is provided at the center of dome, and if mounted on one, two, or more axes gimbals, this optical sensing unit of the prior art can theoretically view a field of regard of, at most, 180 degrees.
Attempts to produce domes in which the FOR is greater than 180 degrees include techniques which separately fabricate two pieces comprising a spherical portion similar to a conventional dome and an extended portion, which are then joined to form the dome. However, the attachment process creates an optical interface along the line of attachment, which produces a discontinuity as the EO seeker scans the FOR. Such a discontinuity poses a risk the seeker may lose track on the target.